Haemorrhagic cystitis of the bladder caused by the antitumour agents cyclophosphamide {2-[bis(2-chloroethyl)amino]tetrahydro-2H-1,3,2-oxazaphosphorine 2-oxide} and ifosfamide [3-(2-chloroethyl)-2-(2-chloroethylami 2-oxide] was studied in the rat. Optimum conditions in this model for protection from toxicity by N-acetyl-l-cysteine were found. Phosphoramide mustard, the ultimate alkylating metabolite of cyclophosphamide, and 5.5-dimethylcyclophosphamide, which is metabolised but forms no cytotoxic products, had minimal effects on the bladder. However, diethylcyclophosphamide [2-(diethylamino)tetrahydro-2H-1,3,2-oxazaphosphorine 2-oxide] caused severe cystitis in the male rat, and significant, but less extensive damage, in the female rat; N-acetyl-l-cysteine protection against this toxicity was demonstrated. As acrolein is the only reactive and cytotoxic metabolite of diethylcyclophosphamide, the role of acrolein as the causative agent in cyclophosphamide cystitis was proven.